This invention relates to a color picture reading system in which the video data of an original are separated according to predetermined hues, to thereby perform the reading operation.
A color reading device for reading color pictures employs a plurality of reading elements or image sensors, for converting optical images of the same original of different wavelength ranges into electrical signals, which are utilized to form video signals separated according to at least two colors.
FIG. 1 shows the arrangement of an optical system of a color picture reading apparatus which reads two colors, namely, red and black. In this apparatus, an original 1 is illuminated by a pair of fluorescent lamps 2 and 3, and light reflected from the original 1 is applied to a half-mirror 4, where it is split into two light beams. One of the two light beams is that reflected by the half-mirror 4. This light beam is converged by a lens 5, so that the image of the original is formed on a first image sensor 6. The other light beam (that which passes through the half-mirror 4) is applied through a lens 7 to a cyan filter 8, where a red wavelength component is removed from the light beam, and the image of the original is formed on a second image sensor 9.
In the first and second image sensors 6 and 9, the optical images are subjected to photo-electric conversion for every scanning line, as a result of which video signals 11 and 12 are provided as shown in FIG. 2. The video signal 11 on the white light side is supplied to a first automatic background control circuit 13 (hereinafter referred to as a first VBC circuit), while the video signal 12 on the cyan light side is supplied to a second automatic background control circuit 14 (hereinafter referred to as a second VBC circuit). The first and second VBC circuits 13 and 14, each being a kind of AGC circuit, operate to allow the background (white) levels to coincide with each other. The video signal 15, the level of which has been adjusted, is applied to a first binary-coding circuit 16 and the plus (+) terminal of a comparator 17. Similarly, the video signal 18 on the cyan light side, the level of which has been adjusted, is supplied to the minus (-) terminal of the comparator 17. In the comparator, the video signals 15 and 18 thus applied are subjected to subtraction to provide a video signal 19, which is applied to a second binary-coding circuit 21.
With respect to pure white, red, blue and black colors, the video signal 15 on the white light side will be in signal level states as respectively indicated from left to right in FIG. 3(a). Therefore, when the video signal 15 is binary-coded with a threshold level V.sub.2 lower than the white level V.sub.1 in the first binary-coding circuit, a video signal 22 is obtained in which red, blue and black signals are in the L(low) level state. On the other hand, the red wavelength component is selectively removed from the video signal 18 on the cyan light side by the cyan filter 8 (FIG. 1). Accordingly, with respect to the above-described color data, the video signal 18 has signal level states as indicated in FIG. 3(b). Therefore, in the video signal from the comparator 17, only the red data is represented by a positive level signal as indicated in FIG. 3(c). Hence, when the video signal 19 is binary-coded with a positive signal level V.sub.4 slightly lower than the red signal level V.sub.3 in the second binary-coding circuit 21, a video signal 23 is obtained in which only red is in the H (high) level state.
The two video signals 22 and 23 thus obtained are supplied to an arithmetic circuit 24 (FIG. 2). In the arithmetic circuit 24, red is separated from the other colors and a final video signal 23 representing the red video data is output as a red video signal 25, or it is output as a red video signal 25 after its logical state is changed. With the video signal 22 representing red, blue and black, the red signal component is removed by way of a logical operation using the video signal 23, so that the video signal 22 is output as a black video signal 26 representing black (including colors other than red). By the utilization of the video signals 25 and 26, the apparatus on the reproduction side reproduces the picture in two colors, namely, red and black.
In the above-described conventional color picture reading system, it is difficult to perform ideal color separation. For instance, in this system in which red and black are separated from one another, blackish red is often read as black, and yellow or orange is read as red. That is, the system is liable to carry out color separation according to a reference which is not satisfactorily in agreement with the human sense of sight, with the result that picture reproduction is unsatisfactory.
The reason for this will be described with reference to FIG. 4. FIG. 4(a) shows the white, red, dark red, light red, yellow (orange), blue and black signal levels of the video signal 15 on the white light side output by the first VBC circuit 13, from left to right therein. FIG. 4(b) indicates the white, red, dark red, light red, yellow (orange), blue and black signal levels of the video signal 18 on the cyan light side as output by the second VBC circuit 14.
As is apparent from a comparison between the video signals 15 and 18, red and yellow (orange) are relatively high in signal level in the video signal 15, and accordingly the difference between the red and yellow (orange) signal levels of the video signal 15 and those of the video signal 18 which has passed through the cyan filter are relatively large. On the other hand, dark red is relatively low in signal level in the video signal 15, and accordingly the difference between the dark red signal level of the video signal 15 and that of the video signal 18 is relatively small. Accordingly, in the video signal 19 as shown in FIG. 4(c), which is the difference between the video signals 15 and 18, light red and yellow (orange) are higher in signal level than dark red. Accordingly, if the video signal 19 is binary-coded with a signal level V.sub.4 which is slightly lower than the signal level of pure red, then slightly dark red which is recognized as red rather than black in the sight sense is read as black, while yellow or orange, which is not so recognized as red, is read as red. If the binary-coding threshold level is decreased in order that dark red is read as red, other colors in the warm color range are read as red. This cannot achieve the object of the reading apparatus that red be accurately read out of the video data.